In rotating machinery, the passages between fixed structures surrounding rotating components provide pathways for leakage of either the working fluid or support system fluids. These pathways may allow fluid from the supporting systems, such as lubricating oil, to leak into the working fluid, or may allow the working fluid to leak into these support systems. These leaks along the shaft, or rotor, of the rotating machinery result in lower operating efficiency and quicker degradation of machine components requiring more frequency maintenance intervals.
To inhibit leakage through these pathways, rotating machines use various seals and sealing techniques. Circumferential seals are commonly used to prevent fluid leakage between compartments. Controlled-gap seals, arch-bound circumferential seals, and segmented circumferential contacting seals are commonly used mechanical sealing methods for circumferential seals. These seals comprise a rotating component, called a seal rotor, sometimes known as a runner, and a non-rotating component called a radial seal or a carbon circumferential seal.
A common configuration is a seal rotor composed of a metallic material and a radial seal composed of carbon (which may also be referred to as a circumferential carbon seal or a carbon circumferential seal). This configuration exhibits a high degree of friction between the rotor and radial seal which wears the carbon seal quickly, resulting in the need for more frequent inspection and replacement. To avoid this friction at the sealing interface, the machine may be designed with a small gap between the metallic seal rotor and the carbon circumferential seal. However, the difference in the coefficients of thermal expansion (CTE), as well as the amount of mechanical growth due to centrifugal effects, between a metallic seal rotor and a carbon circumferential seal is such that it is difficult to maintain this small gap. The carbon circumferential seal and metallic seal rotor will expand at different rates as the machine operating temperature changes and at different machine speeds. Consequently, the gap will either be too large for efficient operation, or will be too small resulting in excessive wear to the circumferential carbon seal.
One solution to this high wear rate is to replace the metallic seal rotor with a ceramic runner. A ceramic runner may be chosen with a CTE close to that of a carbon circumferential seal. Ceramic materials may also experience less mechanical growth from centrifugal effects due to high elastic modulus. The thermal expansion and elastic modulus of ceramic materials allow tighter gaps to be maintained over the operating range of the machine, thereby avoiding some of the above consequences of using a metallic rotor. Additionally, the ceramic material may have a lower frictional force between itself and the circumferential carbon seal. The ceramic material may have a sufficiently low frictional force with the carbon seal that the two may be in contact during operation without significant wear to the seal.
However, the use of ceramic materials imposes challenges in the application of a high temperature machinery, such as a jet engine. A ceramic runner must circumscribe and be affixed, directly or indirectly, to the metallic shaft of the machine. Differences between the CTE of the ceramic and metallic components result in varying stresses on the ceramic component as a result of the differences in thermal growth as temperatures change during machine operations. Additionally, some rotating machines are assembled such that subcomponents are stacked upon one another around the shaft and held together by large compressive forces, a method also known as a lockup assembly. These large compressive forces can create tensile stresses in portions of a ceramic runner. Ceramics may crack under these tensile stresses because they are brittle in nature.